With the deepening of oil and gas exploration and development, there are more and more deep wells, ultra-deep wells and deep-water wells. During the drilling process, it is faced with complex stratum conditions such as multilayered reservoir, multiple sets of pressure systems, high pressure, high sulfur content, cracks, and cavern development. The pore pressure, leakage pressure, and fracture pressure in the stratum are close to each other, and the special working conditions with a narrow safety density window often cause problems such as frequent downhole failures and long drilling cycles, which seriously restrict the oil and gas exploration and development process.
Managed pressure drilling is an adaptive drilling process that precisely controls the entire borehole pressure profile. Through the comprehensive analysis of wellhead back pressure, fluid density, fluid rheology, annulus liquid level, cycle friction and borehole geometry, the hydraulic parameters can be accurately calculated. Real-time adjustment of wellhead back pressure and drilling fluid flow rate is achieved by means of related equipment and processes to control bottom hole pressure within the preset range. The managed pressure drilling technology can effectively prevent accidents such as well leakage and well collapse, increase the mechanical rotation speed, shorten the non-productive time, and at the same time reduce the damage caused by drilling operations to the reservoir.
In the normal drilling process, there are only drilling fluids and cuttings in the annulus, and the back pressure adjustment at the wellhead is relatively easy. When gas invades the wellbore (i.e., gas invasion, the process by which a gas invades the drilling fluid to lower the drilling fluid column pressure and disable the integrity of the wellbore), due to gas phase change, dissolution, slippage, and gas-liquid flow pattern transition, the following problems will be brought to the wellbore pressure precision adjustment system. First, in the case that a well is shut in (which is semi-soft shut-in) due to gas invasion, there is a certain water hammer pressure that may cause damage to the blowout preventer and throttling pipeline and increase the risk of pressure leakage in the exposed stratum. Second, in the case the gas content of the annulus is high after gas invasion, the flow pattern of gas-liquid two-phase is slug flow. There is a significant difference between the flow rate of the liquid slug section and the gas section when passing through the throttling valve and the pressure difference before and after passing through the throttling valve. As a result, there is a periodic large fluctuation in the wellhead back pressure, making it difficult to achieve stable control of wellbore annulus and bottom hole pressure.
When acid gases such as H2S and CO2 intrude into the wellbore, the acid gas is in a supercritical state with a high degree of compression when the depth of the well is deep. When it is close to the wellhead, the phase transition occurs and the acid gases change from a supercritical state to a gas phase. As a result, the density rapidly decreases and the volume rapidly expands.” Due to this behavior of the natural gas with high acid gas content, the gas invasion would be in greater extent hidden in the lower wellbore, accompanied with the abrupt kick in the upper wellbore, which makes it difficult to control the wellhead pressure stably.